Boxing and Unboxing

Boxing- Implicit conversion of a value type (int, char, etc) to a reference type (object) is known as BOXING.

In boxing process, a value type is being allocated on the heap rather than the stack.

Unboxing- Explicit conversion of reference type (object) to a value type is known as UNBOXING.

In unboxing process, boxed value type is unboxed ffrom the heap and assigned to a value type which is being alloccated on the stack.

example: int stackvar = 15;

                Object boxedvar = stackvar; //Boxing = int is created on the heap (reference type)

                int Unboxed = (int) boxedvar; // Unboxing = boxed int is unboxed from the heap

                                                                 // and assigned to an int stack variable.      

example: int i = 5;

                Arraylist arr = new Arraylist(); 

                // Arraylist contains object type value.

                arr.Add(i);n //Boxing occurs automatically.

                int j = (int) arr[0]; //Unboxing occurs.

Issues with boxing and unboxing:

• Sometime boxing is necessary, but avoid it if possible, since it slow down the performance and increase memory requirements.
• Attempting to unbox a null causes a NullreferenceException.

       example:  int? stackvar = null;

                       object boxedvar = stackvar;

                       //NullReferenceException

                        int unboxed = (int) boxedvar;

                       // Object reference not set to an instance of an object.

• Attempting to unbox a reference to an incompatible value type causes an InvalidCastException.

       example:  int stackvar = 15;

                       object boxedvar = stackvar;

                       // InvalidCastException

                       float unboxed = (float) boxedvar; // Specified cast is not valid;